1. Field of the Art
Systems and methods are disclosed for a coherent light generator including an end-pumped laser in which the active media is excited by optical or radiant energy directed through an end surface of the active media, or including a resonant cavity in which laser action takes place by the build-up of electromagnetic field intensity upon multiple reflections. Specifically, novel four-pass end-pump configurations for solid-state lasers are disclosed.
2. Discussion of the Related Art
Directed energy generated by a laser (light amplification by stimulated emission of radiation) is used in many industries for many purposes. Lasers can be used for measuring, spectroscopy, directing, heating, ablating, marking, welding, cutting, pumping other lasers or nonlinear optical crystals, and other uses.
Solid-state lasers typically use a laser gain medium comprising a crystalline solid doped with ions. A common crystalline solid for solid-state lasers is yttrium aluminium garnet (YAG). Dopants include holmium, neodymium, chromium, erbium, ytterbium, cerium, thulium, dysprosium, samarium, and terbium among others. Other laserable media include single crystalline, optical ceramics and glasses. Among single and/or optical ceramic crystal media, other garnet crystals besides YAG can sometimes be equally applicable to embodiments. In addition, crystalline media include yttrium vanadate (YVO4), yttrium aluminate (YAlO3), lithium yttrium tetrafluoride (LiYF4), ZnSe, ZnS, and sapphire. Glasses that can be applicable to embodiments include phosphate, silicate, and fluoride-based glasses.
In a laser, the laser gain medium is “pumped” or otherwise subjected to electromagnetic radiation in order to stimulate the medium to produce laser radiation. An arc lamp, flashlamp, diode, or another laser can be used as a pump, among other devices, to pump a laser gain medium.
Laser radiation is emitted from the laser gain medium when the lasing threshold of the bulk of the medium is reached. The lasing threshold is the lowest excitation level at which a laser medium's output radiation is dominated by stimulated emission rather than by spontaneous emission. Based on an energy level diagram of the laser materials, the lasing process can be understood through the familiar three-level or four-level idealizations. For a three-level laser system, the laser gain medium can be in-band pumped with low quantum defect. However, the three-level laser system can suffer re-absorption loss. In order to overcome the re-absorption loss, a high pump intensity is often required.
End-pumped solid-state lasers have an elongated laser gain medium and are typically pumped longitudinally from one or both ends. End-pumped lasers have been designed such that a pump laser's energy is projected from one end of the laser gain medium to the opposing end. End-pumped lasers have also been designed such that a pump laser's energy is projected from one end of the laser gain medium to the opposing end, which is coated for high reflectance at the pump beam's frequency. What remains of the pump energy at the opposing end is reflected off the end back through the laser gain medium. This double-pass approach results in higher laser extraction efficiency (i.e., laser output power÷pump power). Efficiencies up to 74% have been reported in a double-pass end-pump configuration (see E. Lippert et al., “Midinfrared laser source with high power and beam quality,” Applied Optics, vol. 45, issue 16, pp. 3839-3845 (2006), which is herein incorporated by reference).
For a single end-pumped laser gain medium, the pump intensity is reduced along the longitudinal axis of the laser medium due to absorption by the laser medium of the pump energy. Therefore, the opposite end of the laser medium typically has the lowest pump intensity. Thus, in a single-pass or double-pass end-pumped three-level system, the length of the laser medium must be carefully designed in such a way that the remaining pump energy at the opposite end of the laser medium needs to be sufficiently high to overcome the re-absorption loss, which often cause that the pump energy cannot be thoroughly absorbed by the laser gain medium and subsequently lower the laser efficiency. It is desirable to further increase the efficiency and the laser output power in the three-level laser system.